Magnesium hydride (MgH₂) is a promising material for hydrogen storage applications because of its high gravimetric (7.6 wt.%) and volumetric (110 kg m^-3) densities and low cost. However, there are some drawbacks limited practical application of this hydride. First of all, it is too stable thermodynamically (H=-75 kJ/mol). It results in high desorption temperature (>400ºC). Moreover, the MgH₂ exhibits relatively poor absorption/desorption kinetics below 350ºC. Recently, it has been shown that the sorption kinetics of MgH₂ can be dramatically improved by ball milling of MgH₂ with some transition metals and their oxides or halides. Among them, NbF₅ additive seems most significantly enhance the sorption kinetics of MgH₂. However, it is still not clear how the NbF₅ promotes the dehydrogenation and hydrogenation reactions. Additionally, there is a lack of information about microstructure and sorption properties of MgH₂ with nanosized NbF₅ after cycling loading.
In our work we present the effect of cycling on microstructure and hydriding-dehydriding properties of nanocrystalline MgH₂ with nanosized NbF₅ prepared by mechanical milling. Commercial MgH₂ powder was mixed with NbF₅ powder (7 wt.%) and subsequently ball milled in an inert atmosphere in a planetary mill. The phase structure, morphology and chemical composition were investigated by XRD, SEM, EDS and DSC-TG. The hydrogen sorption properties and pressure composition isotherms were evaluated using a volumetric Sievert apparatus and temperature programmed desorption. Our results have shown a considerable catalytic effect of NbF₅ additive on both the dehydrogenation temperature and hydriding/dehydriding kinetics of MgH₂. The obtained nanocomposite exhibits good reversibility in the pressure composition isotherms at 325ºC. However, degradation of nanostructure and hydrogen storage capacity after prolonged cycling are observed, apparently related to the grain growth during cycling at elevated temperature.

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